Electrophoretic displays are known which incorporate a plurality of parallel cathode lines and a plurality of transverse grid lines insulated from the cathode lines. The cathodes and grids are referred to as rows and columns and the terms can be interchanged. The grid cathode structure forms an X-Y matrix enabling one to address the display at each X-Y intersection to enable pigment particles to migrate to the anode electrode. Such electrophoretic displays have been the subject matter of other prior art patents and essentially the assignee herein, namely CopyTele, Inc. of Huntington Station, New York, has developed many such displays as well as operating techniques for such displays.
Referring to FIG. 1, it is well known to those of ordinary skill in the art that, a display is formed in an electrophoretic display panel 100 as a result of movement of electrically charged particles 110 that are suspended in a fluid 120 which is disposed within a panel structure 100, which panel structure supports the row 130 and column electrodes 140 and the anode electrode 150 structure. The movement of the electrically charged particles is caused by applying potentials to predetermined intersections of the row and column electrodes and to the anode electrode structure to provide predetermined electric fields. As described in U.S. Pat. No. 4,655,897, entitled "Electrophoretic Display Panels and Associated Methods", issued on Apr. 7, 1987 and in U.S. Pat. No. 4,850,819 entitled "Electrophoretic Display Panel Apparatus and Methods Therefor", issued on Jul. 25, 1989, a typical 8.5".times.11" electrophoretic display panel having a resolution of 200 lines per inch comprises approximately 2200 cathode or row electrodes, approximately 1700 grid or column electrodes, and an overlying anode electrode structure.
There is an anode electrode structure which comprises conductor strips instead of a solid thin layer of ITO. This anode structure is described in a copending application entitled DUAL ANODE FLAT PANEL ELECTROPHORETIC DISPLAY, filed on May 1, 1989, Ser. No. 345,825 to Frank J. DiSanto and Denis A. Krusos and assigned to CopyTele, Inc., the assignee herein, which is now U.S. Pat. No. 5,053,763. In an electrophoretic display panel which is used to display characters, characters are formed utilizing a predetermined number of such anode conductor strips in a group, the predetermined number of anode conductor strips being referred to as a character line and each of the predetermined number of anode conductor strips in the character line being referred to as an anode line segment. For example, in a typical such electrophoretic display panel, a character line is comprised of 26 anode line segments, each of which is approximately 0.125" wide and each of which is spaced approximately 0.001" from adjacent segments.
As is well known to those of ordinary skill in the art, an entire electrophoretic display panel which is fabricated in accordance with the prior art described above can be erased by applying a negative voltage to all of the anode line segments in the anode electrode structure. In addition, to provide a "hold" mode of operation or a "write" mode of operation, which modes of operation are described in the U.S. patents identified above, a positive voltage is applied to all the anode line segments in the anode electrode structure. Further in addition, a "selective" erase operation of a multiplicity of character lines, each of which comprises a multiplicity of anode line segments, is achieved by applying a negative voltage to the anode line segments which comprise each of the selected character lines. However, when the above-described "selective" erase operation in such an electrophoretic display panel is utilized to erase a single character line by applying a negative voltage to the anode line segments for the selected character line, a part of adjacent character lines on either side of the selected character line are also erased. Although such partial erasure also occurs when a multiplicity of character lines are erased, partial erasure is particularly unacceptable when few character lines are erased because it produces a display which is illegible and hard to read.
In general, notwithstanding that movement of particles in the electrophoretic display panel is almost perpendicular to the anode and cathode surfaces, some inherent spreading occurs. We have discovered that this inherent spreading causes the above-described erasure of parts of character lines which are adjacent to a character line which is to be erased. Further, we have discovered that such partial erasure of adjacent character lines can be eliminated if the spacing between adjacent anode line segments is increased. However, such a solution is unsatisfactory because the amount of spacing which is required to eliminate the partial erasure is so large that the line structure of the display becomes noticeable to a viewer. The increased spacing also affects resolution as one could not achieve 200 lines per inch without proper line spacing.
As a result of the above, there is a need in the art for an anode electrode structure for a display and, in particular, for an electrophoretic display panel which provides selective erasure of lines without partial erasure of adjacent lines.